Am I missing something obvious?

Got my Garmin 2610 with its Mapsource v8 loaded on my laptop.
Recently purchased a Garmin Forerunner 305 and have tried to install the Training Center (not for me of course, i use it to analyse my sleddogs' training runs).

I can't install the software for the GTC as i keep getting an error message saying there's an installation already in progress. I can't remove Mapsource because I keep getting an error message saying there's an installation already in progress.

Any ideas as to how I can load this?

Run a REGCLEANER of some sort.

What operating system are you using? Sounds like you had an interrupted install at some point.

You could try the Windows Installer cleanup utility:
http://support.microsoft.com/kb/290301